Scientists in NTU have identified promising strategies to improve surface-enhanced Raman scattering (SERS), namely via analyte manipulation and creating hybrid SERS platforms. Headed by Ling Xing Yi at NTU’s School of Physical and Mathematical Sciences, the group has shown that these two techniques are emerging strategies to address limitations found in traditional SERS platforms. These findings were reported in November 2018 in the journal Chemical Society Reviews.
What is Raman spectroscopy, and SERS? Raman spectroscopy is a technique widely used in chemistry to identify molecules via their vibrational fingerprints, relying on inelastic scattering of photons, known as Raman scattering. However, the Raman signals of most molecules are very weak. By using nanostructures made of silver, gold, copper and aluminium, SERS enhances these signals by up to 108-folds. Thus, SERS is gaining attention as a spectroscopic technique that can detect and identify trace molecules using their unique molecular vibrational fingerprints.
Figure 1: Surface-enhanced Raman scattering (SERS) and its underlying principles. (a) Scheme depicting the use of noble metal nanostructures to support localized surface plasmon resonances (LSPRs) and the corresponding electromagnetic mechanism for drastic Raman signal enhancement during SERS. SERS signal provides molecular vibrational fingerprint essential to identify analyte species at the molecular level. (b) A two-pronged approach to achieve ultrasensitive SERS readout by integrating analyte manipulation techniques with hotspot engineering.
A vial containing silver nanostructures
SERS is widely used in investigations in chemical reactions, clinical diagnosis, toxin sensing, and environmental/food/industrial surveillance. However, it has its limitations – target analytes must be close to the metallic nanostructures to access the electromagnetic hotspots for enhancement. Hence, it remains a formidable challenge to detect analyte such as gases that do not adsorb onto the SERS platforms.
This is where the Ling research group’s contributions become important: With analyte manipulation and hybrid materialsm, promising strategies are emerging to address the limitations mentioned earlier.
The main advantage of analyte manipulation (over the conventional hotspot engineering) is better sensitivity and broader analytical applications for detection of important yet weakly adsorbing molecules. These strategies mainly work by capturing and confining analytes into electromagnetic hotspots where field enhancement and corresponding SERS intensities will be the strongest.
Effective analyte manipulation allows SERS to be employed in wide applications including medical diagnosis, e.g. SERS detection of antibody immunoglobulin G from a complex blood matrix can be achieved at fg/mL concentrations. In another instance, the use of designer nanostructures with analyte-capturing moieties enables the direct quantitative sensing of chemical warfare stimulants e.g. methylphosphonic acid (MPA) down to 1 ppb, which has implications in national security, defense and anti-terrorism.
In hybrid SERS platforms, secondary functional materials are incorporated with conventional metallic nanoparticle-based SERS platforms, including graphene and semiconductors. This boosts SERS sensitivities, potentially improving the detection limits by 10 to 100 times as compared to conventionally metallic nanoparticle-based SERS platforms.
However, these approaches are still in their infancy and have challenges that need to be addressed.
At the same time, these strategies lead to a paradigm shift beyond hotspot engineering, which is vital to ensure continuous improvement of SERS platforms with regards to their sensitivity and practicality. By combining the research advances and benefits of individual scientific/engineering disciplines, we can anticipate thriving research in such multi-faceted approaches to lead to a rapid breakthrough in the design of next-generation hybrid SERS platforms.
Authors of the paper. Absent: Lee Hiang Kwee, Lay Chee Leng and An Qi
Reference:
Hiang Kwee Lee, Yih Hong Lee, Charlynn Sher Lin Koh, Gia Chuong Phan-Quang, Xuemei Han, Chee Leng Lay, Howard Yi Fan Sim, Ya-Chuan Kao, Qi An and Xing Yi Ling, Designing surface-enhanced Raman scattering (SERS) platforms beyond hotspot engineering: emerging opportunities in analyte manipulations and hybrid materials, Chem. Soc. Rev, 2019, 48, 731-756